A film composed of a fluororesin, such as a polytetrafluoroethylene (PTFE), has features, such as superior chemical resistance and heat resistance, and for example, a fluororesin film processed to have pores therein is used as a filter which filters fine particles. When the fluororesin thin film is used as a filter, in order to achieve filtration and segregation of fine particles and to obtain superior filtration ability (filtering characteristics), the film is required to have a smaller thickness as well as minute and uniform pore diameters and a high porosity.
The fluororesin thin film processed to have pores therein can be obtained, for example, by stretching a resin film of PTFE. According to a method for stretching a non-porous fluororesin thin film, a porous fluororesin thin film having minute pore diameters can be manufactured.
Since PTFE has a high melt viscosity, a thin film cannot be formed by melt extrusion or the like. Accordingly, in general, there have been used a method in which a PTFE resin powder is mold into a cylindrical block, followed by shaving off the surface of the block, and a method in which a paste formed by mixing a PTFE resin powder and a liquid lubricant is molded by extrusion, followed by rolling or the like. However, since the methods described above are each a process by applying a mechanical load, when the film thickness is decreased, in particular, to 20 μm or less, due to an insufficient strength of the film against the mechanical load, the film may be stretched, wrinkled, bored, or broken in some cases; therefore, it has been very difficult to obtain a thin film having a thickness of approximately 20 μm or less.
As another method, there has been conceived a method (casting method) in which a dispersion composed of a liquid and a fluororesin powder dispersed therein is applied on a base material and is then heated to a melting point or more for sintering the fluororesin powder as well as for removing the liquid. For example, in Japanese Unexamined Patent Application Publication No. 5-32810 (Patent Document 1), a method for forming a thin film has been disclosed in which after a dispersion of PTFE powder particles is applied on a heat resistant substrate and is then heated to a melting temperature or more of PTFE to bind the powder particles to each other. The thin film thus formed is peeled away from the substrate and is then stretched, thereby forming a porous PTFE thin film.